1. A continuing effort involves studies of a defective MuLV (BM5def) responsible for induction of murine AIDS (MAIDS) whose only product is a Gag protein which differs from the Gag polyprotein of a nonpathogenic ecotropic virus in the carboxyterminus of MA(p15) and throughout much of a p12. In an attempt to understand how these altered p15 and p12 contributes to the development of lymphoproilferation an immunodeficiency we made several attempts to generate mice expressing BM5 def-Gag from a transgene. These studies, where Gag sequences were driven by SV40 promoter and IgH Eu enhancer or MHC class II E alpha promoter, yielded mice bearing the transgenes, but without any expression. More recently, after several rounds of unsuccessful infections with BM5def genomic DNA, higher frequency transfer was achieved. Sufficient numbers of mice are now available for studies of expression of these genes and also for challenge with replication competent ecotropic MuLV as well as the LP-BM5 mixture of viruses from which BM5def was originally isolated. 2. Although genomes of xenotropic and polytropic (MCF) viruses are present in many copies in all inbred strains of mice, the presence of ecotropic viral genomes is variable (0-5 copies) indicating a recent introduction of these viral genomes in inbred mice. Ecotropic viruses isolated from wild-mouse populations of various regions of the world are different structurally, whereas, ecotropic viruses isolated from inbred strains are very similar, if not identical, as measured by restriction endonuclease mapping analyses. A search for the presence of ecotropic virus genome present in the inbred populations from the wild lead us to identify a region in Denmark where the DNAs of the wild mouse population harbors one copy of this ecotropic virus genome.